Production of chromium compounds



Patented June 1 1, 1946 s PATENT OFF ICE ' PRODUCTION OF CHROMIUM COMPOUNDS Marvin J. Udy, Niagara Falls, N. Y.

No Drawing. Application March 19, 1942,

Serial No. 435,438

6 Claims.

This invention relates to the production of hexavalent chromium compounds and has for an object the provision of improved methods of extracting hexavalent chromium from crude materials containing chromium in the hexavalent state combined with oxygen and a base in the form of chromate. More particularly, the invention contemplates the provision of improved methods of extracting hexavalentchromium from crude materials containing chromium in the form of calcium chromate. The invention further contemplates the provision of improved methods of producing calcium chromate and sodium dichromate. I

Chromate for industrial uses is prepared and marketed largely as sodium dichromate. The chromate is produced initially as sodium chromate but conversion to the dichromate is desirable because of difliculties involved in obtaining a suitable solid sodium chromate product for marketing. Chromate in the form of calcium chromate would be desirable if calcium chromate could be produced. and recovered directly from crude chromium-bearing materials at a recovery cost of the order of the cost of recovering sodium chromate, since calcium chromate contains relatively cheap lime (CaO) as the basic constituent, whereas sodium chromate contains relatively expensive sodium oxide as the basic constituent, and calcium chromate can be crystallized in the form of desirable solid products. No commercially satisfactory procedure for producing and recovering calcium chromate has been proposed by others heretofore. Accepted commercial practices for producing both calcium chromate and-sodium dichromate involve the use-of sodium chromate as the source of chromate radical.

These procedures have been adopted because sodium chromate can be produced readily by direct oxidation of chromium-bearing materials, and, being very soluble in water, it can be separated readily from solid impurities, whereas, sodium dichromate can not be produced readily by direct oxidation of chromium-bearing materials, and calcium chromate, being relatively slightly soluble in water, is difllcult to separate from solid impurities. Both procedures result in the wasting of sodium'oxidc oi the sodium chromate. The production and marketing of chromate as sodium dichromate rather than as calcium chromateis explained by the fact that in the production of sodium dichromate one-half of the sodium oxide of the sodium chromate is utilized, only'one-hali being wasted, and no additional base is required, whereas, in the production of 2 calcium chromate, all of the sodium oxide or the sodium chromate is wasted and additional base i the form of lime (CaO) in amount equivalent to the sodium oxide of the sodium chromate must be provided.

1 Calcium chromate can be produced q i ea y as ferrochromium, oxidized ferrochromium and chromite are in the presence of .lime or in the presence of lime and a small amount of an oxidation promoter such as soda ash. Its recovery from products obtained in such roasting operations is rather difllcult and expensive because of its relatively low solubility in water. Therefore,

calcium chromate is produced, according to some present practices, by first producing sodium chromate and thereafter treating the sodium chromate in aqueous solution with a soluble calcium compound to form the relatively insoluble calcium chromate which is :ecovered by evaporation and crystallization.

Procedures involving the production of calcium chromate by first producing sodium chromate may be wasteful of reagents. The production of the sodium chromate involves roasting of chromium-bearing' material with air in the presence of a sufilcient amount of a sodium compound such as sodium carbonate to form sodium chromate with all of the chromium of the chromium-bearing material. The conversion of all of the chromium to sodium chromate may require the use of a substantial excess of the sodium compound which excess may be wasted, or it may be recovered at considerable expense. Conversion of the sodium chromate to calcium chromate by treatment of the sodium chromate in aqueous'solution with a soluble calcium compound results in the production of a sodium compound of the type of soluble calcium compound employed, and the sodium compound thus formed is practically worthless commercially. Thus, for example. sodium carbonate may be employed in the roasting operation to produce chromate. Any excess sodium carbonate employed at this'point is wasted as a. result of combining with one or more components of the chromium-bearing material to form insoluble compounds. The sodium chromate solution, after concentration. may be treated with calcium chloride to form calcium chromate in accordance with the following equation:

The sodium chromate and calcium chloride react to .form the relatively slightlysoluble calcium chromate. The chromium ofthe sodium chroby roasting chromium-bearing materials such also, an alkali metal clu' 3 a mate can be obtained largely as calcium chro mate by repeated evaporation and crystallization treatments. The sodium chloride produced is wasted because the commercial value of sodium chloride is so low that its recovery in a suitably pure commercial formwould not be economical. Thus, the process results in a substantially complete loss of thesodium compound employed in the initial roasting treatment. 7

When sodium dichromate (NazCiuOr) is to be prepared in accordance with some heretofore customary procedures, chromium-bearing material is roasted as described above to produce, sodium chromate (NazCrOO, a sodium chromate solution is formed by leaching the roasted marterialwith water, and sodium dichromate "is compounds less soluble in water than calcium chromate or the compounds may be free of constituents capable of combining with calcium to form compounds less soluble in water than calcium chromate. Whenthe compounds containing. the bases as constituents are free of constituents capable of combining with calcium to form formed by treating the sodium chromate'soluv tion with an acid such assulphuric acid. Sodium chromate and sulphuric acid react to produce sodium dichromate and sodium sulphate as indicated by the following equation:

The sodium sulphate and-the sodium dichromate may be separated by'evaporation and crystallization, the sodium sulphate being wasted.

The present invention provides methods of pro- I ducing calcium chromate by means of which the aqueou liquid containing in solution an acid or an acid salt capable of reacting with calcium chromate to effect its decomposition and to form a compound of calcium less soluble in water than calcium chromate. The composition 01 the pulp preferably is so regulated as to effect conversion to, dichromate of the chromium of the decomposed calcium chromate. Conversion of the chromium of the decomposed calcium chromate to dichromate is controlled by establishing in the pulp a controlled amount of one or more bases capable o1 combining with chromate ions to produce one or more soluble dichromate compounds.

The bases employed may be metallic elements or radicals which function as metallic elements in forming chemical compounds. Thus, for example, the bases which are suitable for use in practicing the invention include calcium (Ca), sodium (Na), potassium (K) and the. ammonium radical (NHO The bases may be employed in the form of any suitable saltsor other compoimds, that is,

in chemical combination with non-metallic elements or with base or acid forming radicals. The compounds containing the bases as constitua process of the invention for 45 from crude chromate-bearing material containing calcium chromate, the crude chromate-bearing material is digested in the form of a pulp with an compounds less soluble in water than calcium chromate, they are employed in conjunction with acids having constituents which are capable of combining with calcium to form compounds less soluble in water than calcium chromate. Thus, for example, the base may be provided in the form of an acid carbonate or acid sulphate which compounds are capable of reacting with calcium chromate to form compounds (calcium carbonate and calcium sulphate) less soluble in water than calcium chromate, or,'the base maybe provided in the form '01 pound should be employed in conjunction with an acid, suchv as carbonic acid or sulphuric acid,

capable of combining with calcium to form a compound less soluble in water than calcium chromate.

Processes of the invention are based on reactions of the following types:

In processes employing the above reactions, calcium chromate of the crude calcium chromatebearing material provides the base for combining with the chromate ionsto form the soluble dichromate compound, calcium dichromate, and carbonic acidformed by reaction of carbon -dioxide with water provides the acid constituent capable of combining with calcium to form a ater than calcium chromate.

gompound of calcium (CaCOs) less soluble in In processes employing the above reactions, calcium chromate is decomposed by sodium acid carbonate with the production of highly soluble sodium dichromate and slightly soluble calcium carbonate.

In processes employing the above reactions, sodium acid carbonate formed by reaction 0! carbonic acid with sodium chromate probably forms-the active agent for decomposition of the-calcium chromate as in Equation 2 above.

Crude calcium chromate-bearing material for treatment in the process of the invention may be obtained from any suitable source. Such material may be produced, for example, by roasting in air ferrochromium or chromite ore in finely divided condition and in the presence of lime to ents may be salts of acids whose acid radicals I are capable of combining with calcium to form oxidize elemental chromium or trivalent chromium contained therein to the hexavalent state with the production of calcium chromate. The roasting charge may containan oxidation promoter such as sodium carbonate or other sodium compound in which case sodium chromate may also be formed, The amount of sodium chromate produced in any roasting operation will depend upon the relative quantities oi! sodium and chromium present in the roasting charge. When sodium dichromate or calcium chromate is the end product sought, roasting charges should be prea chromate in which case the compared with sodium present in amounts not exceeding that required to form'sodium chromate with half of the chromium present and lime should be provided in amounts suiiicient to form calcium chromate with the remainder of the chromium.

when sodium dichromate is the end product sought. roasting preferably is so conducted as to form a product containing sodium chromate and calcium chromate in about equi-molecular proportions. The resulting product may be digested in an aqueous pulp with carbon dioxide to convert substantially all of the chromate to sodium dichromate in accordance with the following equamate-bearing material. When sodium chromate 6 4 treated with calcium hydroxide as described above to form calcium chromate and sodium chromate. Sodium chromate thus formed may be separated from the calcium chromate. and recovered as such or converted to sodium dichromate or it may be used in conjunction with carbon dioxide to treat additional calcium chrosolution is employed to treat calcium chromatebearing material containing sodium chromate,

.it is advisable to bleed from the leach solution containing sodium dichromate 'an amount of so- V HaaCnOr+CaCOa+HaO The liquid containing in solution the sodium diroasting charge in an aqueous pulp with carbon dioxide to produce a solution of calcium dichromate and treating the solution of calcium dichromate' with calcium hydroxideto form calcium'chromate in accordance with the following equations: I I

(5) 2CaCrOt+C0z+HzO- CaCraO1+CaCO:+HaO

(e) oacnoq+cuomr+zcscroi+rno The relatively insoluble calcium chromate thus formed may be precipitated by boiling or evapcrating the solution. Anhydrous calcium chroq mate may be precipitated Jrom solution by heating the solution to a temperature of about 180 C. or higher under pressure of about 150 pounds per square inch or higher. 4

when some'sodium chromate is formed inoperations designed for the production of calcium chromate, the roasted charge may be digesteddirectly with carbon dioxide and water with the production of a mixture of sodium dichromate and calcium dichromate; it may be leached first with water to remove the soluble sodium chromate and then digested with water and carbon dioxide to produce calcium dichromate; or it may be digested with carbon dioxide and a solution of sodium chromate to miO uce a solution of sodium dichromate. The solution of sodium dichromate may he treated with calcium hydroxide to form calcium chromate and sodium chromate .inaceordance with the following equation:

(7) NaaCnOr+Ca(OH)-.--

The calcium chromate thus formed may be precipitated from solution in the hydrated state or the anhydrous state, and the solution of sodium chromate may be separated from the precipitate and used for leaching additional calcium chromate-bearing material. A solution of calcium dichromate may be treated with calcium hydroxide as described above to produce calcium naicrol- -cacroi-tmo dium dichromate equivalent to the sodium chromate contained in the calcium chromate bearing material to be treated before returning it to the process.

' When carbon dioxide and water are employed to digest calcium chromate-bearing material in order to produce calcium dichromate, digestion preferably is carried out under-pressure andat an elevated-temperature. Grinding of the calcium chromate-bearing material during digestion may be carried out advantageously. Grinding prevents accumulation of precipitated calcium carbonate on the surfaces of particles of calcium chromate with resulting reduction in the rate of solution of the calcium chromate. Pressure; of 40 pounds per square inch and higher may be employed advantageously. An elevated temperature of about 70 C. or higher prevents the formation an solution of calcium bicarbonate (camwonn, which forms at lower temperatures but which decomposes at temperatures above about 70 C. Digestion may be carried out at lower temperatures and the solution containing calcium dichromate and calcium bicarbonate may be heated after separation from the solid material to decompose the calcium bicarbonate.

Calcium carbonate produced by decomposition of 4n the calcium bicarbonate may be separated from chromate. A solution containing calcium dichromate and sodium dichromate may also be the calcium dichromate solution by filtering.

when sodium chromate is employed in con- Junction with carbon dioxide and water for leaching calcium chromate-bearing material, lower pressures and temperatures may be employed satisfactorily. The reactions take place rapidly at atmospheric pressure.

I claim: e

i. The method of recovering chromium from chromium-bearing material which comprises subjecting the material to an oxidizing treatment in the presence of lime and soda ash to form an oxidized product containing calcium chromate and "sodium chromate, digesting the oxidized product in' the form of a pulp with an aqueous liquid containing in solution sodium chromate and carbon dioxide to decompose calcium chromate and form sodium dichromate and insoluble calcium carbonate, the composition of the pulp being so regulated as to etlect conversion to sodium dichromate. of a substantial amount of the chromium of the decomposed calcium chromate, separating a sodium dichromate solution from insoluble material of the oxidized product, treating the sodium dichromate solution with a calcium compound under pressure at a temperature not lower than aboutv C. to form a solution of sodium chromate and precipitate anhydrous calcium chromate, separating the solution of sodium chromate and the anhydrous calcium chromate precipitate, and utilizing the solution of sodium chromate in the digestion of additional calcium chromate-bearing material.

-2. The method of recoveringchromium from w chromite ore which comprises subjecting the ore g? to an oxidizing treatment in the presence of lime and soda ash to form on oxidized product contaming calcium chromate and sodium chi-ornate, digesting the oxidized product in the form of a. pulp with an aqueous liquid containing in solution sodium chromste and carbon dioxide to decompose culclum chromute and form sodium dichromete and insoluble calcium carbonate, the composition of the pulp being so regulated us to effect conversion to sodium dichrom to of 2. substsntial amount of the chromium of the decom posed calcium chromote, separating as sodium di chromete solution from insoluble material of the oxidized product, treating the sodium dichrornute solution with e, calcium compound under pressure at e temperature not lower than about 180 C. to form e. solution of sodium chi-ornate and preclpitate anhydrous calcium chromate, separating the solution of sodium chromcte and the cnhy drous calcium chromete precipitate, and utilizing the solution of sodium. chromste in the digestion v of additional calcium chromatic-bearing material. 3. The method of recovering chromium irom ferrochromium which comprises subjecting the ferrochromium in finely divided form to an oxidizing treatment in the presence of lime and soda ash to form an oxidized product containing calcium chromate and sodium chromate, digesting the oxidized product in the form of a pulp with an aqueous liquid containin in solution sodium chromete and carbon dioxide to decompose calcium chromete and form sodium dichromate and insoluble calcium carbonate, the composition of the pulp being so regulated as to efiect conversion to sodium dichromcte of a. substantial amount of the chromium of the decomposed calclum chromate, separating e sodium dichromate secures with calcium hydroxide, heating the solution to e temperature not lower then about 180 it, under pressure to form and precipitate slnhydrous cel= cium chromete, and separating the anhydrous calcium chromcte from the accompanying liquid.

5. The method of producing anhydrous cel= cium chromste which comprises trectina an aqueous solution containing calcium dichromote with calcium hydroxide, heating the solution to a temperature not lower than about let Q. under pressure to form and preclpitete anhydrous calcium chromate, and separating the anhydrous calcium chromate from the accompanying liquid.

6. The method of producing anhydrous calcium chromate which comprises treating an aqueous solution containing a dichromate of the group consisting of sodium dichromate and colclum dichromste with calcium hydroxide, hectirig the solution to a. temperature not lower than shout 180 C. under pressure to form and precipitote anhydrous calcium chrome-to, end seperuting the anhydrous calcium chromete from the accompanying liquid.

MARVE? J. UJDY. 

